In a typical image forming apparatus, an electrostatic image is formed on an image forming member, which may for example be the photoconductive surface of a rotating drum or the photoconductive surface of a moving belt. The electrostatic image is, for example, formed by charging the photoconductive surface to a first potential V.sub.0, known as the "dark" potential, and then image-wise exposing the charged photoconductor surface to dissipate the charge on image areas. The electrostatic image is brought into the vicinity of a developing device, which is supplied with developer, typically a mixture of a particulate toner and magnetic carrier particles.
It is common practice to apply the toner-carrier mixture to the surface carrying the electrostatic charge image by means of a developing unit wherein toner and magnetizable carrier particles are mixed and a layer of the toner-carrier mixture, referred to herein as "developer", is picked up by an applicator such as a rotating sleeve or drum having magnets inside, forming a so-called magnetic brush on a "magnetic roller".
In one type of development unit toner particles are mixed with larger magnetizable carrier particles, to which the toner particles adhere by electrostatic attraction force. The electrostatic charge of the toner and carrier particles is obtained triboelectrically by agitation. The charge sign of the toner particles is opposite to the charge sign of the carrier particles.
On rotating the magnetic roller, the toner particles still adhering to the magnetically attracted carrier particles are brought into a developing zone wherein the toner particles are separated from the carrier particles by the electrostatic attraction forces of the electrostatic latent image to be developed and transfer to the latent electrostatic charge image. The sign of the toner particles, compared with the sign of the charge on the image forming member, determines whether the development is a "direct" or "reversed" development. If the toner and the image forming member have opposite signs, the development is direct; toner particles will be attracted to the charged areas of the image forming member. If the toner and the image forming member have the same sign, the development is "reverse"; toner particles will be attracted to the discharged areas of the image forming member.
A DC developing bias potential V.sub.DC of suitable value is applied between the magnetic brush and the back electrode of the image forming member. The sign of the DC bias potential is the same as that of the image forming member. The value of the DC bias potential is typically between the value of the potential of the image areas and that of the non-image areas.
The term "cleaning potential" is defined as the absolute value of the difference between the potential of the non-image areas and the DC bias potential. The main effect of this cleaning potential is to establish an electric field between the magnetic roller and the image forming member at the non-image areas which repulses the toner particles away from the image forming member back to the magnetic brush.
The term "development potential" is defined as the absolute value of the difference between the potential of the image areas and the DC bias potential. The main effect of this development potential is to establish an electric field between the magnetic roller and the image forming member at the image areas which attracts the toner particles to the image areas.
Toner particles are attracted to the electrostatic image on the image forming member to thereby form a toner image. Subsequently the image forming member, carrying the toner image, comes into contact with a substrate, for example paper in sheet or web form, to which the toner image is transferred. Alternatively, the transfer of the toner image from the image carrying member to the substrate may be by way of one or more intermediate transfer members.
It is known to superimpose an AC voltage over the DC bias between the developer carrying member and the back electrode of the image forming surface.
This AC development method has a number of advantages. Higher toner amounts can be transferred towards the photoconductor during AC development than can be achieved with DC-only development, resulting in higher print densities on the image. Using an AC electric field during development reduces the development time constant considerably, resulting in a better development of image areas containing a sharp transition from a high density to a low density or vice versa. The result is an image with sharper well-defined image edges. The image density developed with AC development is less sensitive to variations in distance of the photoconductor to the magnetic roller, and less sensitive to variations in developer supply on the magnetic roller. Furthermore, AC development leads to images with less blow-off and a better homogeneity of line widths.
An example of an image forming apparatus using AC development is shown in U.S. Pat. No. 5,314,774 (Hewlett Packard) which describes a method and apparatus for developing and printing color images on a moving photoconductive belt. A number of developing devices are spaced from the belt and are AC and DC biased to project toner onto the belt. The composite color image thereby formed on the belt is then transferred to an intermediate belt and from there to a final substrate. A relationship is disclosed defining the motion of toner particles in the air gap between the developer carrying member in the developing device, and the belt in terms of the size of the toner particles, the viscosity of the air gap, the charge on the toner and the DC and AC electrostatic fields.
European patent specification EP 432998-A (Xerox Corporation) describes a scavenge-less/non-interactive electrostatographic development system in which a powder cloud is generated between a developer donor roller and a set of wires mounted between the donor roller and an image forming belt. For use in highlight color imaging, the system uses the combination of an AC voltage on the donor roller with an AC voltage between the toner cloud forming wires and the donor roller to control the developability of lines and the degree of interaction between the toner and a photoconductive belt.
U.S. Pat. No. 5,409,791 (Eastman Kodak Company) describes an image forming method in which an electrostatic image on an photoconductive belt already carrying a loose dry first toner image is toned with a second toner, for example having a different color. The toning is accomplished by a developer having a high coercivity permanently magnetized carrier and toner which is moved through a development zone by a rapidly rotating core inside a sleeve on which the developer moves. Scavenging of the first toner image is prevented by separating the sleeve from the photoconductive belt sufficiently that the crests of the developer do not touch the photoconductive belt during the toning process. An alternating electrical field is applied between the sleeve and the photoconductive belt to enhance development.
A problem which arises with AC development onto photoconductor belts, especially where the photoconductor is an organic photoconductor, is background development, especially when AC development is used in combination with a high belt speed. It appears that the higher surface roughness which is typical of belt photoconductors, as compared with drum photoconductors, contributes to this problem.
It is an object of the present invention to provide a method of AC development, in which the image is substantially free of background.